A photo diode for monitoring intensity of laser light is integrally attached to some laser light source represented by a semiconductor laser. In accordance with this laser light source, since it is possible to dynamically control a driving electric current with monitoring an output signal from the photo diode, the intensity of the laser light can be kept stably for a long period of time, irrespective of a characteristic fluctuation due to an ambient environmental change or aging.
Meanwhile, in case that a gas is measured by the use of the laser light output by the laser light source, the stability of the intensity of the laser light is also important, however, furthermore, it is necessary to make the center oscillation wavelength of the laser light stably coincided with the peak absorption wavelength of the gas. If the oscillation wavelength of the laser light deviates from the peak absorption wavelength of the gas, the intensity of the light of the gas absorption wavelength in the wavelength component of the laser light rapidly decreases because the wavelength band of the laser light is extremely narrow, thereby significantly lowering the analysis accuracy.
Then conventionally, as described in the patent document 1, a part of the laser light is introduced into a cell that contains a gas of the same kind as that of the analysis object gas and the intensity of the light passing the cell is measured so as to verify whether or not the oscillation wavelength of the laser light is kept accurately.
In addition, an infrared spectrophoto analyzer by the use of the laser light source as shown in the patent document 2 stabilizes an optical path by mechanically assembling optical systems with multiple optical elements positioned accurately.    Patent document 1: Japan patent laid open number 2001-21493    Patent document 2: WO95/26497